I think there is a thread on this matter from a year or so ago.
The preservation argument re: the old 9th hole is the most persuasive, but really, how many of any people can tell us what happened historicaly on that hole during the Opens played there? It's all conjecture that "history" was made on that green, unless you trivialise what history really means.
Lot's of real golfing history has taken place on places that no longer exist in their present form. It may be sad to some, but it's life, which does move on.
There is a real NIMBY element to this story. Let's keep our twee little course and our mid-20th century vistas at the expense of more (and probably better) golf for the people of Mussleburgh as well as economic regeneration of the area through the expanded race course. And, the historical precendents are on the "other" side, if anything. For example:
--should Muirfield have been prohibited from burying decades of golfing history when they hired Harry Colt to over-write Tom Morris' work in the 1920s?
--Should Tom Morris himself have been prohibited from making the changes he did to the 18th green of the Old Course when he was keeper of the green, overwriting the history of the previous Opens held there?
BTW, Tommy, Tom MacKenzie is a very nice guy. If you disrespected him after meeting him, the problem would be yours, and not his or ours.